In recent years, the use of communication systems having wireless mobile units which communicate using an optical or radio link with a hardwired network, such as a local area network (LAN), has become quite widespread. Retail stores and warehouses, for example, may use such systems to track inventory and replenish stock. Employees may enter inventory information using a handheld or portable mobile communication unit which can be carried throughout the store or warehouse. In manufacturing facilities, such systems are useful for tracking parts, completed products and defects. In a medical environment, these systems can reduce the time needed to fill out forms and eliminate inaccuracies by allowing medical personnel to transmit data directly from a mobile communication unit carried by the medical personnel.
Referring to FIG. 1A, an example of a conventional communication system is designated 20 and generally includes a number of fixed access points 21 interconnected by a cable medium 22 to form a hardwired network. The hardwired network, known as a backbone, may be a twisted pair cable, shielded coaxial cable or fiber optic lines, for example. Each access point 21 has a cell or service area which is defined as the area surrounding the access point 21 within which it has the ability to transmit and to receive relatively error-free data from a mobile communication unit MU within the area. In many instances, the cell is not circular because physical objects within the cell may partially block data signals creating "dead spots" within the area surrounding the access point 21. Also, machinery located in the vicinity of the access point 21 may generate excessive noise levels that degrade the quality of transmissions between the access points 21 and the mobile communication units MU.
In such a network, a mobile communication unit MU (also referred to herein as a "mobile unit") communicates with a host computer 36 by wirelessly transmitting a packet of data to the access point 21. The access point 21 then transmits the packet on the hardwired network to the host computer 36. The host computer 36 communicates with the mobile unit MU by transmitting a packet to the access point 21 to which the mobile unit MU is considered to be registered. The access point 21 then wirelessly transmits the packet to the mobile unit MU. The mobile unit MU communicates to the network through one access point 21 at a time. This access point is the access point 21 with which the mobile unit is registered. Because the mobile unit MU may move from cell to cell, however, it generally is able to change its access point registration.
When a mobile unit MU is powered up, it "registers" with an access point 21. However, as the location of this mobile unit MU changes, the mobile unit MU may register with another access point 21, thereby resulting in a deregistration with the previous access point 21. Further, deregistration will sometimes occur if there is no communication between the mobile unit MU and its corresponding access point 21 within a predetermined period of time.
Because the mobile unit MU may roam from cell to cell and change its access point registration, the network protocol must provide a system 20 for assuring that a packet addressed to the mobile unit MU is sent to the correct access point 21 for broadcast to the mobile unit MU. To provide this assurance, current networks utilize a non-source routed protocol.
For example, FIG. 1A shows a typical non-source routed Ethernet network. Communication over such a network occurs when a network device places a packet of data (also referred to herein as an information packet, or simply a packet) on the network. Network devices can place information on the Ethernet any time there is no other communication occurring. When a packet is placed onto the network, every device on the network will simultaneously receive the packet and view the destination address or addresses. The destinations that are specified in the packet will copy the packet into their memory for further processing. All other devices on the network will ignore the packet.
Each access point 21 on the Ethernet system maintains a registration table indicating which mobile units are currently within the service area of the access point. Given that every access point 21 has the opportunity to view all packets placed onto the network, each access point 21 is able to check its registration table and determine whether a specific mobile unit MU is currently registered. If a mobile unit MU roams from cell to cell, only the access point 21 with which the mobile unit MU is currently registered to will copy the frame and transmit it to the mobile unit MU. Thus, continuous communication with a roaming mobile unit MU is possible.
There may be instances while roaming that a mobile unit MU is not currently registered to any access point. In some Ethernet access points, a mail forwarding routine has been developed to handle such situation. However, such mail forwarding routine involves an access point with which a mobile unit MU was previously registered storing packets intended for the mobile unit. When the mobile unit MU later registers with a new access point, the access point with which the mobile unit MU was previously registered simply places the stored information back onto the bus from which it was received. There is no rerouting of the stored information.
The problem associated with non-source routed networks such as the system 20 shown in FIG. 1A is that sending all packets to all possible destinations is inefficient and reduces total throughput in a heavily loaded system. Therefore, source routed networks have been developed.
For example, in a source routed network such as the multiple token ring network shown in FIG. 1B, packets are not regularly transmitted to every device on the network. Rather, the source routing field in the packet designates the path a packet should take in order to reach its final destination. In particular, the source routing field consists of an ordered list of ring and bridge numbers which set out the entire path needed for a given packet to reach its destination from a source device on the network.
The problem with a communication system having a source routed network occurs when the system is used in conjunction with wireless mobile units MU. In particular, if a mobile unit MU roams from an area serviced by a first access point 21 on a first token ring to an area serviced by a second access point 21 on second token ring, packets addressed-to this mobile unit MU may still be routed to the first access point 21 on the first token ring. For example, typically the host computer 36 on the source routed network as shown in FIG. 1B will be configured to transmit packets to the mobile units based on one or both of the following approaches. In one case, the host computer 36 has stored therein a fixed position table indicating the location of the various rings on the network. In the event a mobile unit MU transmits a packet to the host computer 36 via an access point 21 on a given ring, the host computer 36 will respond to the mobile unit MU by directing a packet back to the same ring based on the location information in its position table. Alternatively, the host computer 36 may simply reverse the order of the source routing information in the packet received from the mobile unit MU. In either case, however, the destination ring of the packet transmitted by the host computer 36 is based on the ring location of the access point 21 that transmitted the packet for the mobile unit MU. Since the mobile unit MU may have since moved, this ring may not always be the correct location to which packets should be transmitted. Thus, the destination may not be able to maintain effective communication with the mobile unit MU to which the packet is directed.
Many establishments already have source routed networks such as that shown in FIG. 1B installed to perform a variety of functions. It would be very expensive and time consuming to reconfigure the system such that network devices which may be transmitting information to the mobile units MU are kept apprised of the location of each of the mobile units MU so as to avoid the loss of data. This is particularly true with respect to the host computer 36 which typically is the heart of the system and would need significant reprogramming and reconfiguration. The same may also be true with respect to the other devices on the network which may need to communicate with the mobile units MU and access points 21 such as a controller, cash register, etc. Accordingly, there is a strong need in the art for a network system, particularly one utilizing source routing, which does not require extensive reconfiguration of the host computer or other network devices which may need to communicate with the mobile units and access points. In particular, there is a strong need in the art for a means in which such systems can quickly and inexpensively be configured to ensure that packets addressed to a mobile unit reaches the mobile unit independent of which access point the mobile unit is currently registered.